sniffbooger

Hi I have a stock 1994 Civic VX 1.5l SOHC (Vtec-E) hatch and it runs perfect in 1st and 2nd gear... all temperatures, all speeds, all RPMs. No hesitation, no lag, no stumbling. But soon as I shift to 3rd-5th gear, it starts losing power, surging and stumbling after a couple seconds. It will lose 3-4 MPH, then it will accelerate back up to speed and repeat the process. When the engine is cold, it does not do this. My guess is something to do with the closed loop fuel monitoring and/or fuel mixture because of the lean burn mode. But WHY does it run like poo poo only in 3rd-5th gear, and after its warmed up???

Notes:
Engine runs fine when cold in all gears.
CEL does not come on at any time.
I have already tried disconnecting the EGR valve and it does not remedy the problem.
New Walker O2 sensor was installed last week but does not remedy the problem.
ECU was reset.
I tried disconnecting IAT sensor and it threw CEL and ran so/so in default mode but idle was rough so I re-connected IAT.

83gs1100g

WOW, I have the same issue on my 92 vx??? When cold its all good! After warmed up enough to go into lean burn is when miss starts? New o2, plugs, wires, distributor, injectors. Idle holds steady on the tach but still has a miss and shakes only when warm? Only thing left is to drill intake ports to see if there clogged? I tried pulling egr off, start car and spray half a can of carb cleaner in the egr intake port. This helped a little? Any more ideas??? Oh, no cel's and plugs are burning perfect!

sniffbooger

I have currently been running the car with the IAT sensor disconnected so it will run in default mode with the CEL on and so now it only gets about 40mpg but at least the horrible surging is gone at cruising speeds. Any suggestions would be helpful.

83gs1100g

hook the iat back up, reset ecu, then pull each plug wire one at a time to see that the engine idles down some, if it doesn't idle down or very little then thats your bad cylinder? if all idle down the same then check your timing?

sniffbooger

Update:. After owning three of these (revised to say NON) California VX models with the 5 wire O2 sensor, I figured out what causes the loss of power at cruising speeds. (Revised to say 49 State VX models I am referring to in this post.)

Before the engine warms up, it runs richer in open loop mode. After it warms up, it will enter closed loop at cruising speeds causing a lean burn air fuel ratio approx 17 to 1 give or take. This is by design for fuel economy with the VTEC-E motor (NON-California model only).

Under acceleration, the ECM will switch back to open loop (richer) for power. This system works fine when the engine is new or has low miles, and can yield mpg as high as 60+ with hypermiling.

However, most of my old VX hatchbacks had 150-225k on the original engine....and they were burning oil. That is what causes the loss of power when the car enters closed loop (lean) mode. It is burning just enough oil to cause the power loss and inability to maintain a steady speed. The O2 sensor is reading the exhaust as richer than it actually is, sending an improper air fuel ratio to the ECM...and this causes the problem. Not much I could do to fix it, other than rebuild the engine or keep the intake air temp sensor disconnected...that way it always stayed in open loop (rich) mode and made it more drive able. However, the cons were the CEL staying on, and the mpg usually stayed around 40-42hwy. Still not bad for a 25+ year old car!

tony_2018

these vtec-e engines are mainly for mpg. If you try to extract any power out of it you're not getting any. If you boost you can probably do it efficiently and gain more mpg. All comes down to the tune.

TomCat39

Good to know.

The engine really doesn't do as well once high mileage sets in. Thanks for sharing.
I bet that bugged you for a good long time.... Like 9 years. :D

deschlong

iTrader: (8)

No doubt ... thank you for posting back all this time later. As an owner of a car with a D15Z1, this is helpful to know!

Relic1

FYI: California VX's
92-early 93 - NGK L1H1 Wideband O2, Standard P07 ECU with lean burn disabled. (P07-L00 or P07-L01 designation)
late 93-95 - standard narrow band O2 - variant of the P28 (P07-L02 designation)

if you have a 94 VX with a L1H1 wideband either it's not cali, or it's been converted. (the ECU will tell you)


Do not underestimate a good valve lash adjustment. (I do not see that mentioned) It made the world of difference in my Z1.

**edit**
ok the L01/L02 may be the trust in my poor memory playing tricks on me. I'm not 100% that there was a L01 in the Wideband controlling version, that may be the narrowband version.
It's been a while since I held one.

sniffbooger

Thank you for bringing this to my attention...I made a typo and have revised my post. I meant to say the NON-California VX models that have the 5 wire O2 sensor and the lean burn mode in my post. These are the 49 State, NON California models that don't run well in lean burn mode when the engine starts burning oil. FWIW